Corrosion inhibitor containing arsenous oxide and potassium hydroxide



Patented Apr. 21, 1953 UNITED STATES ATENT OFFICE CORROSION INHIBITOR CONTAINING AR- SENOUS OXIDE AND POTASSIUM HY- DROXIDE tion of Delaware N Drawing. Application December 22, 1951, Serial No. 262,995

4 Claims. 1

This invention relates to a corrosion inhibitor for use in inhibiting the corrosion of ferrous metal piping and tubing in producing wells and in pipe lines transporting crude oil. More particularly, it relates to a solid arsenous corrosion inhibitor.

The corrosion of ferrous metal tubing and piping in producing oil wells and in pipe lines transporting crude oil is now and has for some years been a serious operating problem confronting the petroleum producer. Corrosion of tubing and piping necessitates frequent interruptions of production to pull corroded tubing and piping from the well and replace them with new material. The essential features of a corrosive environment very commonly encountered in producing oil wells include a ferrous metal in contact with oil, brine, and gas containing carbon dioxide, gas liquid interfaces in contact with the metal, and areas of turbulent liquid flow in contact with the metal surfaces. The metal in this environment is corroded away by direct attack of carbonic acid which is greatly accelerated by electrochemical phenomena arising out of the contact of the phase interfaces with the metal and out of the contact of turbulent flowing liquid with the metal surface. Substantially all of the acidity of the production stream stems from the presence of carbon dioxide. The acidity is quite low, the pH not being lower than 3 and being ordinarily in the range 4 to 6. When a ferrous metal is exposed to the production efliuent under quiescent conditions where there is no movement of the liquid or of free gas bubbles through the liquid, the corrosion rate observed is usually less than about 25% of that observed under actual producing conditions which include turbulent motion of the liquid in contact with the metal and the presence of minute gas bubbles which contact the metal. In laboratory apparatus in which the separate influence of the three corrosive factors was studied, 1. e., direct acid attack, electrochemical attack attributable to turbulent liquid flow, and electrochemical attack attributable to the presence of finely-divided gas bubbles which contact the metal surfaces, it was found that the relative rate of corrosion due to direct acid attack was 4 units, and that the relative rate of corrosion when either the condition of liquid turbulence or finely-divided gas bubbles moving through the liquid was superimposed on the presence of the acid medium, the relative corrosion rate immediately increased to 14 units. From these observations it is clear that the coriosion problem would not be solved by the employment of an inhibitor which would eliminate only the corrosive action due to direct acid attack and that the corrosion problem here presented can be solved only if the additional electrochemical corrosion attributable largely to physical conditions is markedly reduced. The corrosion inhibitor of this invention is especially well adapted to minimizing thecorrosion of ferrous metals in the environment and by the mechanisms described.

A variety of physical setups is found in producing oil wells which determine the manner of introduction of the corrosion inhibitor into the well. Some wells have open annuli and a corrosion inhibiting solution can be lubricated into the well through the open annulus. Some wells have packed-off annuli and the corrosion inhibitor must be introduced through the production tubing by pumping an inhibitor solution to the well bottom and permitting it to flow upward with the production stream or by dropping a solid corrosion inhibitor through the tubing to the well bottom and producing the well. In condensate wells, where there is no liquid production stream, the corrosion inhibitor to be effective must in some manner be applied to the interior surface of the tubing without the aid of a liquid production stream to carry and spread it A solid corrosion inhibitor which can be placed at or near the well bottom by dropping it through the well tubing can be utilized effectively in all types of wells if the solid inhibitor has the following described properties. It must be an effective inhibitor at low concentrations. Only relatively small amounts of an inhibitor can be introduced into a well by dropping the solid inhibitor through the tubing. Some commonly used inhibiting chemicals such as dichromates and caustic soda will inhibit satisfactorily if used in large amounts, but these materials are not suitable for injection into the well in solid form since the maintenance of the desired concentration requires very frequent interruption of the production for the introduction of further amounts of the inhibitor. A suitable chemical inhibitor for injection into the well through the tubing in solid form would be one that provides protection against corrosion at concentrations of about one to five parts per million in the produced water; an effective solid corrosion inhibitor should have a high persistency in its inhibiting effect. To be practical, the injections of a solid inhibitor should not be made more often than once in twenty-four hours, and preferably once 3 in several days, or once a week. High persistency of the inhibitor or inhibiting film formed on the metal surfaces of the tubing is highly desirable in solid inhibitor injection. A successful solid corrosion inhibitor should be highly soluble in well water. This is particularly desirable if the inhibitor is to be used in a condensate well. Relatively insoluble or highly insoluble materials could easily cause plugging of the tubing. A solid corrosion inhibitor should have a high density so that the solid inhibitor will fall through the production stream while the well is being produced. If the solid inhibitor has this property, the production need be interrupted for only the few minutes necessary to introduce the solid into the tubing. The solid inhibitor, if it is to be effective in a condensate well, should soften at the temperature existing at the zone of condensatlon. Condensate wells produce little or no formation water and much of a water-soluble chemical that falls below the condensate zone will not be returned with the production. If the solid inhibitor softens or melts at temperatures from about 150 to 200 F., the solid will be spread on the hot tubing wall as it falls inthe well.

Arsenous compounds, particularly the alkali metal arsenites or slurries of arsenous oxide with alkali metal hydroxides and water, meet the above-described requirements with respect to effectiveness at low concentration, persistence, and solubility admirably. Particular arsenous compositions hereinafter described meet not only the first three requirements, but also the requirements with respect to density and to softening point in a remarkable manner.

'It has now been found that a hard, homogeneous solid corrosion inhibitor can be prepared by intimately mixing arsenous oxide, potassium hydroxide and water, and cooling the resultant mixture to remove exothermic heat of reaction. The mixture contains potassium hydroxide and arsenous oxide in amounts such that the ratio of potassium hydroxide to arsenous oxide is above 1.221, and preferably in the range 1.221 to 6.521. The water content ofthe mixture is from 1 to 3.5 parts by weight to each parts by weight of potassium hydroxide and arsenous oxide together and is smaller within that range as the KOH/ASzOs ratio is lower within the ratio range 1.2:1 to 65:1. An organic hydroxy compound such as ethylene glycol and other similar materials set forth hereinafter is desirably substituted forpart of the water in the composition.

Compositions containing these ingredients in these proportions arehard at atmospheric temperature, have a density above about 2.5- grams per cubic centimeter, and are thermoplastic in the sense that they soften at temperatures from about. 150 to about 250- F., and finally becom liquid at higher temperatures.

Numerous compositions consisting essentially of arsenous. oxide, potassium hydroxide and water were prepared and their properties. at atmospheric, temperature were examined. The composition and properties of several of these mixtures are summarized in Table I.

From the data in the above table it is clear that variation of the proportions of arsenous oxide, potassium hydroxide and water in the compositions affect the hardness of the mixture at atmospheric temperature.

The properties of arsenous oxide, potassium hydroxide and water mixtures were further studied in order to define the boundaries of the hard-setting compositions. It was found that the mixtures are hard setting when the ratio of potassium hydroxide to arsenous oxide is greater than 12:1 and when the water content of the mixture lies in the range 1 to 3.5 parts by weight to each 15 parts by weight of potassium hydroxide and arsenous oxide combined. It was found impractical to attempt to reduce the water content below 1 part by weight, since it was virtually impossible to make a homogeneous mixture of the three components before the partially mixed materials had set.

Since it is the arsenous oxide component which is the primary corrosion inhibitor material in the composition, mixtures having potassium hydroxide to arsenous oxide ratios above 6.5:1, although hard setting, are not very desirable for the intended use because a large amount of the material must be introduced into the well tubing in order to have sufficient arsenous oxide present to do a satisfactory inhibiting job. To keep the volume of material introduced down, it is preferred to use mixtures having potassium hydroxide to arsenous oxide ratios from 4:1 to.1.2:l. As the ratio of KOI-I to AS203 rises in the range 1.2:1 to 65:1, the amount of water that can be added to 15 parts by weight of the mixed solids and still produce a hard setting mixture increases in the range 1 to 3.5 parts by weight. In order to facilitate mixing of the components, it is preferred to employ at least-1.5 parts by weight of water to each 15 parts by weight of potassium hydroxide and arsenous-oxide combined; to insure softening points above F., it is preferred not to use in excess of 3 parts by weight of water even at a 6.5:1 potassium hydroxide to arsenous oxide ratio and if potassium hydroxide to arsenous oxide ratios from 4:1 down to 12:1 are employed, the water content is preferably below 2.5 parts by weight. e

Data obtained from observatlon'of additional arsenous oxide-potassium hydroxide-water mixtures useful as solid corrosion inhibitors are set forth in TabIe II.

The softening temperature recorded in Table II is the temperature at which the mixture exhibited a putty-like consistency and could be molded by manual pressure.

The softening point of the compositions changes abruptly when the potassium hydroxide to arsenous oxide'ratio is reduced below 12:1. The softening points of the hard-setting compositions can be controlled in a considerable degree by varying the water content from 1 to 3.5 parts by weight to each 15 parts by weight of potassium hydroxide and arsenous oxide together, the softening point being lower as the water content is increased. a 1

- The solid inhibitor compositions above described may be improved in two respects by substituting an organic hydroxy compound such as ethylene glycol for a part of the water. The substitution of these materials for part of the water very considerably reduces the rate at which the compositions set up as hard solids when the three ingredients are mixed. When arsenous oxide, potassium hydroxide and water are mixed in the proportions above described, they tend to set very rapidly. This characteristic becomes a serious problem in commercial production of the inhibitor compositions where it is desirable to mix large batches of the inhibitor and pour the mixture into molds to set. The setting is so rapid that the compositions set up as solids in the mixing vessel before pouring can be completed. The substitution of ethylene glycol for part of the water greatly reduces the setting rate and thus facilitates commercial production of the solid inhibitor cast in forms suitable for introduction into the well. The substitution of ethylene glycol for a part of the water in the inhibitor compositions increases the resistance of the solid product to shearing and impact forces. When water alone is used, the solid tends to be somewhat grainy and brittle and, when it is cast in stick form, the stick may be rather readily broken by shearing or impact force. This characteristic is undesirable if it is intended to drop the solid inhibitor in stick form into the well tubing without any outer casing. The sticks strike against the tubing as they fall and may be broken into small pieces which are carried up by the production stream before reaching the well bottom.

Substitution of ethylene glycol for to 50% by weight of the water brings about noticeable improvements in the compositions both with respect to the speed of setting and with respect to the resistance of the solids to impact and shearing forces. The softening temperatures of the compositions are not adversely affected by the substitution of ethylene glycol in these amounts. The preferred organic hydroxy compound for use in the inhibitor compositions is ethylene glycol. However, it has been determined that other organic hydroxy compounds will perform the same function, for example, aliphatic alcohols containing up to 12 carbon atoms per molecule, diethylene glycol and triethylene glycol produce similar effects on the setting time and resistance to fracture of the inhibitor compositions.

The inhibitor compositions above described are desirably cast in the form of pellets or sticks. The most usual form is a cylindrical stock from 1 to 3 feet in length and having a diameter from 1 to 2 inches, that is, a diameter generally adapted to permit the fall of the stick through conventional well tubing. The sticks ordinarily weigh from 2 to 9 pounds and have a density above about 2.5 grams per cubic centimeter. Their mass and density are such that they readily fall through the production stream of the average producing well so that the production need be interrupted only for the few minutes which are required to introduce the stick into the well tubing. The introduction of such a stick every three to ten days will ordinarily bring the corrosion rate of a well down to an acceptable low level and maintain it there.

Since the arsenical compounds are poisonous, the sticks are desirably wrapped or coated with some non-toxic material in the interest of safe handling. The wrapping or coating may be either functional or non-functional and may be an integral part of the stick as it is dropped into the well, or may be removable at the time of injection.

When the well to be treated is a flowing well discharging a production stream of oil, brine, and carbon dioxide, a metal having a standard oxidation reduction potential above 0.5 and below 2.5 volts, such as magnesium, aluminum, zinc, and their alloys, forms an excellent functional integral coating for the inhibitor. Corrosion inhibiting cartridges can be prepared by pouring the molten corrosion inhibitor into a thin walled (0.01 to 0.05 inch) metal tube or cylinder and permitting it to solidify there. The ends of the tube or top of the cylinder are then closed. The closure can be made by capping with a metallic cap, or crimping the tubing, or sealing with a high-melting point wax or asphalt. The metal lic coating is an integral part of the inhibitor cartridge as it is dropped into the well and the metal is functional in its action in the well and not merely a container for the inhibitor. Magnesium, for example, slowly dissolves in dilute carbonic acid contained in the well water and, in dissolving, neutralizes part of the acid; in addition, metals such as magnesium, zinc, aluminum and their alloys provide cathodic protection against electrolytic corrosion.

Other integral coatings which are non-functional may be employed such as a high-melting point wax or an oil-soluble or water-soluble plastic material. The solidified inhibitor stick can be dipped in these materials and allowed to dry and the surface film of these materials provides protection to personnel against handling hazards. In the well the protective coating is rapidly dissolved, placing the inhibitor composition in contact with the production stream.

The inhibitor sticks may also be protected by an outer container which is removed prior to the introduction of the inhibitor stick into the well. The hot, soft corrosion inhibitor is poured into paper cylinders which maintain their shapes when the temperature of the inhibitor reaches or exceeds its softening point during shipment or storage. The paper tubing is peeled away from the stick before dropping it into the well tubing. It has been found that if the hot corrosion inhibitor mixture is poured into untreated paper tubes, some difficulty is experienced in attempting to remove the paper from the hardened stick. However, if the paper is greased with a fairly heavy grease, or covered with a waterimpervious coating such as shellac or plastic, prior to the introduction of the molten inhibitor composition into the paper tube, the paper may easily be removed from the solidified stick. Alternatively, the molten inhibitor can be poured into a rubber cylinder and allowed to solidify there. When the inhibitor is solid, the top of the rubber cylinder is pressed together and sealed with rubber cement.

The corrosion inhibitors of this invention have been tested in flowing wells discharging oil, brine, and carbon dioxide and have proven highly effective. In the average well good corrosion control is maintained by introducing two magnesium cartridges per week into the tubing. The magnesium cartridges are 2 feet long, 1 inches in diameter, and contain 5.5 pounds of the solid inhibitor, the proportions of the components in the solid inhibitor being 5.5 parts by weight of arsenous oxide, 9.5 parts by weight of potassium hydroxide, and 1.8 parts by weight of water.

We claim:

1. A solid corrosion inhibitor consisting essentially of an intimate mixture of arsenousoxide, potassium hydroxide and water, the ratio by weight of potassium hydroxide to arsenous oxide in the mixture being greater than 1.2:1 and the water content of the mixture being from 1 to 3.5 parts by Weight to each fifteen parts by weight of potassium hydroxide and arsenous oxide together, the water content being lower at lower potassium hydroxide-arsenous oxide ratios.

2. A solid corrosion inhibitor consisting essentially-of an intimate mixture of arsenous oxide, potassium hydroxide and water, the ratio by weight of potassium hydroxide to arsenous oxide in the mixture being in the range 1221 to 65:1 and the water content of the mixture being from l'to '3 parts by weight to each fifteen parts by weight of potassium hydroxide and arsenous oxide together, the water content being lower at lower potassium hydroxide arsenous oxide ratios.

3. A solid corrosion inhibitor consisting essentially of an intimate mixture of arsenous oxide, potassium hydroxide, ethylene glycol and water, the ratio by weight of potassium hydroxide to arsenous oxide in the mixture being in the range 1.2:1 to 65:1, the combined weight of ethylene glycol and water contained in the mixture being from 1 to 3 parts by weight of potassium hydroxide and arsenous oxide together and the weight of ethylene glycol in the mixture being less than of the weight of ethylene glycol and water together. 7

4. The method of inhibiting corrosion in a producing oil well delivering a production stream comprising petroleum, brine and carbon dioxide gas which comprises periodically interrupting the production. dropping a solid corrosion inhibitor formed by intimately mixing arsenous oxide, potassium hydroxide and water, the ratio by weight of potassium hydroxide to arsenous oxide in the mixture being greater than 12:1 and the water content of the mixture being from 1 to 3.6 parts by weight to each 15 parts by weight of potassium hydroxide and arsenous oxide together, through the well tubing and producing the well.

GILSON H. ROHRBACK. DWITE M. McCLOUD. WILLARD R. SCOTT, JR.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,638,710 Sherbino Aug. 9, 1927 1,877,504 Grebe et a1. Sept. 13, 1932 

1. A SOLID CORROSION INHIBITOR CONSISTING ESSENTIALLY OF AN INTIMATE MIXTURE OF ARSENOUS OXIDE, POTASSIUM HYDROXIDE AND WATER, THE RATIO BY WEIGHT OF POTASSIUM HYDROXIDE TO ARSENOUS OXIDE IN THE MIXTURE BEING GREATER THAN 1.2:1 AND THE WATER CONTENT OF THE MIXTURE BEING FROM 1 TO 3.5 PARTS BY WEIGHT TO EACH FIFTEEN PARTS BY WEIGHT OF POTASSIUM HYDROXIDE AND ARSENOUS OXIDE TOGETHER, THE WATER CONTENT BEING LOWER AT LOWER POTASSIUM HYDROXIDE-ARSENOUS OXIDE RATIOS.
 4. THE METHOD OF INHIBITING CORROSION IN A PRODUCING OIL WELL DELIVERING A PRODUCTION STREAM COMPRISING PETROLEUM, BRINE AND CARBON DIOXIDE GAS WHICH COMPRISES PERIODICALLY INTERRUPTING THE PRODUCTION, DROPPING A SOLID CORROSION INHIBITOR FORMED BY INTIMATELY MIXING ARSENOUS OXIDE, POTASSIUM HYDROXIDE AND WATER, THE RATIO BY WEIGHT OF POTASSIUM HYDROXIDE TO ARSENOUS OXIDE IN THE MIXTURE BEING GREATER THAN 1.2:1 AND THE WATER CONTENT OF THE MIXTURE BEING FROM 1 TO 3.5 PARTS BY WEIGHT TO EACH 15 PARTS BY WEIGHT OF POTASSIUM HYDROXIDE AND ARSENOUS OXIDE TOGETHER, THROUGH THE WELL TUBING AND PRODUCING THE WELL. 